Atmospheric Sciences & Global Change Division Research Highlights

From Earth to Table to Air

Based on U.S. crop production, scientists determined what regions take in more carbon than they release (blue), and what regions release more carbon than they take in (red). Enlarge Image

Results: A potato growing in Idaho uses carbon as a building block, but after it is enjoyed at a Los Angeles bistro, that spud's carbon is released into the sunny California atmosphere. Research by scientists at Pacific Northwest National Laboratory, Oak Ridge National Laboratory, and Colorado State University provides a detailed account of agricultural carbon's journey from farm to market, illustrating how our regional interdependence for agricultural production shifts carbon impact. The study was published in the journal Biogeosciences.

Why it matters: The natural carbon cycle is complex, and modern agriculture and transportation make it mobile. Crops in the United States take in and later return about 37 percent of the nation's total annual carbon dioxide emissions, but that amount varies by region. Understanding how and where agricultural CO2 is stored and released can help shape climate change policy, as decision-makers seek new strategies to reduce greenhouse gas emissions.

"Until recently, climate models have assumed that the carbon taken up by crops is put back into nature at the same place crops are grown. Our research provides a more accurate account of carbon in crops by considering the mobile nature of today's agriculture," said Dr. Tristram West, a scientist at the Joint Global Change Research Institute, a partnership between PNNL and the University of Maryland.

Methods: During photosynthesis, plants take in carbon dioxide and convert it into carbon-based sugars needed to grow and live. When a plant dies, it decomposes and releases carbon dioxide into the atmosphere. Or, after eating fruits and vegetables, animals and humans release the plants' carbon as either carbon dioxide while breathing or as methane during digestion.

While the total carbon taken in by plants is about the same as is released in the end, the rise of commercial agriculture has shifted the balance of where carbon is released. Crops are harvested and shipped far away from where they're grown. As a result, agriculturally active regions take in large amounts of carbon as crops grow. These regions are called carbon sinks. Meanwhile, regions with larger populations consume those crops and release the carbon. They are carbon sources. These differences are important for understanding the potential impacts of greenhouse gases.

To accurately measure agricultural carbon, researchers looked at 17 crops that comprise 99 percent of total U.S. crop production, including corn, soybeans, wheat and cotton. They calculated the carbon content of harvested crops by county from 2000 to 2008. Population numbers and data on human food intake were used to estimate, by age and gender, how much carbon from crops people consume and release through bodily functions. They did the same analysis on livestock and pets, and also accounted for crops lost to spoilage. Beyond food, they calculated the amount of carbon that goes into plant-based products, such as fabric, cigarettes and biofuels. And they noted how much grain is stored for future use and the crops that are exported overseas.

"These calculations substantially improve what we know about the movement of carbon in agriculture. Reliable, comprehensive data like this can better inform policies aimed at managing carbon dioxide emissions," said West.

What's next: The research team plans to apply their methods to forestry, which also involves the movement of carbon-containing products from one region to another. With this data, comprehensive carbon calculations for both agriculture and forestry can be integrated into climate models.